Email was designed and used at a time when the Internet was still a safe and friendly place. This is no longer the case, yet many email servers still allow anyone on the Internet to access their services to send spam email, and many of us still send our account passwords and email messages in clear text (think postcard versus envelope) over the wire.
Many email programs have the ability to sign and encrypt email messages. This allows us to verify the identity of the sender of a received email message, verify that the message has not been tampered with in transit, and, finally, send encrypted email messages. Simply put, it offers the ability to put our email messages in an envelope and seal them. More.